Catalyst for the selective oxidation of sulfur compounds to elemental sulfur, process for preparing such catalyst and process for the selective oxidation of sulfur compounds to elemental sulfur

ABSTRACT

The invention relates to a catalyst on support for the selective oxidation of sulfur-containing compounds to elemental sulfur, comprising at least one catalytically active material applied to a support material, this catalyst being obtainable by applying the catalytically active material to a support material which comprises at least one alkali metal promotor.

The invention relates to a supported catalyst for the selectiveoxidation of sulfur compounds, in particular hydrogen sulfide, toelemental sulfur, a process for preparing such catalyst and a processfor the selective oxidation of hydrogen sulfide to elemental sulfur.

The necessity of purifying gases which are further treated in chemicalprocesses, supplied to buyers, or discharged to the atmosphere, fromsulfur compounds, in particular hydrogen sulfide, is generally known.Accordingly, a number of processes are known which are directed to theremoval of hydrogen sulfide from gas.

One of the best-known methods of converting hydrogen sulfide tonon-harmful elemental sulfur is the so-called Claus process.

In the Claus process, however, the H₂ S is not quantitatively convertedto elemental sulfur, mainly as a result of the equilibrium character ofthe Claus reaction:

    2 H.sub.2 S+SO.sub.2 →2 H.sub.2 O+3/n S.sub.n       ( 1)

A residual content of H₂ O and SO₂ remains. Now, generally it is notallowed to discharge H₂ S-containing residual gas, and so the gas mustbe combusted, whereby the hydrogen sulfide and other sulfur compounds aswell as the elemental sulfur present in the gaseous phase are oxidizedto sulfur dioxide. As the environmental requirements are becomingstricter, this will not be allowed anymore because the resultant sulfurdioxide emission would be too high. It is therefore necessary to furthertreat the residual gas of the Claus plant, the so-called tail gas, in aso-called tail gas plant.

Tail gas processes are known to those skilled in the art. The best-knownand to date most effective process for the treatment of tail gas is theSCOT process (See GB-A-1,461,070). In this process the tail gas,together with hydrogen, is passed over a cobalt oxide/molybdenum oxidecatalyst applied to Al₂ O₃ as a support, whereby the SO₂ present iscatalytically reduced to H₂ S. The total amount of H₂ S is thenseparated in conventional manner by liquid absorption. One drawback ofthe SCOT process is that it requires a complicated plant. Anotherdrawback is the high energy consumption involved in removing thehydrogen sulfide from the absorbent again.

Another option in converting hydrogen sulfide in tail gas to elementalsulfur is the so-called BSR Selectox process, disclosed in U.S. Pat. No.4,311,683. According to this process, the H₂ S-containing gas, mixedwith oxygen, is passed over a catalyst containing vanadium oxides andvanadium sulfides on a non-alkaline, porous, refractory oxidic support.

An important drawback of both the SCOT process and the Selectox processis that in both cases the tail gas, after hydrogenation of the sulfurcomponents present to H₂ S, must first be cooled to remove the greaterpart of the water, because water greatly interferes with the absorptionand the oxidation of H₂ S. Due to the associated high investmentsinvolved, the costs of tail gas treatments according to these knownmethods are high.

Another process for the oxidation of H₂ S to elemental sulfur isdisclosed in U.S. Pat. No. 4,197,277. According to this publication, thehydrogen sulfide-containing gas is passed with an oxidizing gas over acatalyst which comprises iron oxides and vanadium oxides as activematerial and aluminum oxide as support material. Further, the supportmaterial, which has been impregnated with the active material, has aspecific surface larger than 30 m² /g and a pore volume of 0.4-0.8 cm³/g, while at least 12.5% of the total pore volume is constituted bypores having a diameter greater than 300 Å. It has been found that thiscatalyst gives rise to at least a partial Claus equilibrium, so that theformation of SO₂ cannot be prevented. As a result, the effectiveness ofthis process is insufficient. The effectiveness with respect to theconversion of H₂ S to elemental sulfur may generally be adverselyaffected by the occurrence of the following side reactions:

1. the continued oxidation of sulfur:

    1/n S.sub.n +O.sub.2 →SO.sub.2                      ( 2)

2. the reversed (or rather reversing) Claus equilibrium reaction:

    3/n S.sub.n +2 H.sub.2 O<->2 H.sub.2 S+SO.sub.2            ( 3)

Here the sulfur, once it has been formed, enters into a reverse reactionwith the water vapor also present to form hydrogen sulfide and sulfurdioxide.

The occurrence of the side reactions mentioned above is partlydetermined by practical conditions.

In general tail gas comprises, in addition to elemental sulfur, aconsiderable amount of water vapor, which amount may be within the rangeof 10-40% by volume. The water vapor strongly promotes the reversingClaus reaction. The substantial removal of water vapor has evidenttechnical disadvantages, such as the necessity of an additionalcooling/heating stage, an additional sulfur recovery stage or ahydrogenation stage followed by a water-removing quench stage. A processin which the conversion to elemental sulfur is not influenced by thewater content of the feed gas is therefore desirable.

Another important circumstance is that generally in the selectiveoxidation some excess of oxygen will be used not only to prevent the H₂S from "slipping through" but also on the ground of considerations ofcontrol technology. This very excess of oxygen, however, may give riseto the continued oxidation of the elemental sulfur formed, therebyadversely affecting the effectiveness of the process.

U.S. Pat. No. 4,818,740, which is incorporated herein by reference,discloses a catalyst for the selective oxidation to elemental sulfur,the use of which prevents the above side reactions to a large extent,while the main reaction ##EQU1## takes place with a sufficient degree ofconversion and selectivity.

The catalyst according to that patent comprises a support of which thesurface exposed to the gaseous phase does not exhibit any alkalineproperties under the reaction conditions, while a catalytically activematerial is applied to this surface. Further, the specific surface areaof this catalyst is less than 20 m² /g and less than 10% of the totalpore volume in this catalyst has a pore radius in the range of 5-500 Å.

An improvement of the method disclosed in the above-mentioned U.S. Pat.No. 4,818,740 is disclosed in European patent publication 409,353, whichis incorporated herein by reference. This patent publication relates toa catalyst for the selective oxidation of sulfur-containing compounds toelemental sulfur, comprising at least one catalytically active materialand optionally a support, which catalyst has a specific surface area ofmore than 20 m² /g and an average pore radius of at least 25 Å, whilethe catalyst exhibits substantially no activity towards the Clausreaction under the reaction conditions.

It has been found that the catalyst according to this European patentpublication gives a clear improvement in the activity and selectivity ofthe catalysts. In spite of this, there remains a need for an improvementof the catalyst in order to increase the yield of elemental sulfur.

The object of the present invention is to provide a catalyst for theselective oxidation of sulfur compounds to elemental sulfur, which showsa higher conversion of the sulfur-containing compounds to elementalsulfur.

The invention relates to a catalyst on support for the selectiveoxidation of sulfur-containing compounds to elemental sulfur, comprisingat least one catalytically active material applied to a supportmaterial, which catalyst is obtainable by applying the catalyticallyactive material to a support material which comprises at least onealkali metal promoter.

Surprisingly, it has been found that such a supported catalyst shows animproved conversion of sulfur-containing compounds to elemental sulfur.In this connection, it is essential that the alkali metal promoter ispresent in the support. It has been found that this can be realized onlyby providing the catalytically active material on a support whichcontains the alkali metal promoter. This can be effected by applying thealkali metal promoter during the preparation of the support or duringthe shaping thereof (extrusion, tableting, granulation, etc.). It isalso possible to impregnate the support with a solution of the alkalimetal promoter prior to the application of the catalytically activematerial. A simultaneous application of the two components does not leadto an improvement of the elemental sulfur yield.

According to the invention, therefore, it is essential that the alkalimetal promoter is not applied to the catalyst simultaneously with orafter the active component. It has been found the activity of thecatalyst does not improve or may even deteriorate if the promoter isapplied to the catalyst simultaneously with or after the activecomponent.

The catalysts according to the invention can have a specific surfacearea which can vary within wide limits. Starting from the given that thecatalyst preferably exhibits substantially no activity towards the Clausreaction under the reaction conditions, a skilled artisan can determinethe desired specific surface area. This surface area also depends on thenature of the support, a smaller surface area being desirable foraluminum oxide supports than for silicon oxide supports.

According to one embodiment, a catalyst according to the invention has aspecific surface area of more than 20 m² /g and an average pore radiusof at least 25 Å. Such a catalyst preferably has silicon oxide as asupport.

According to another embodiment of the invention, the catalyst has aspecific surface area of less than 20 m² /g and less than 10% of thetotal pore volume in this catalyst has a pore radius between 5 and 500Å. With such a catalyst, it will be preferred to start from aluminumoxide as support.

It is noted that in the present invention the absence of Claus activityis defined as the absence of the influence of water on the selectivityof the oxidation reaction of H₂ S to sulfur in the presence of minimallya stoichiometric amount of O₂ at 250° C. More particularly, this meansthat in the presence of 30% by volume of water the selectivity of thereaction to elemental sulfur should not be more than 15% lower than theselectivity in the absence of water. This definition of the Clausactivity is based on the equilibrium Claus reaction

    3/n S.sub.n +2 H.sub.2 O<->2 H.sub.2 S+SO.sub.2            ( 3)

If a material is Claus active, the presence of water results in thereaction proceeding in the direction of H₂ S and SO₂, whereby a part ofthe sulfur is converted to H₂ S and SO₂ again. H₂ S is then oxidizedwith the O₂ present to sulfur and water vapor, whereafter the Clausactive catalyst converts the sulfur to SO₂ again. Due to the concurrenceof these reactions a catalyst with Claus active sites will, in thepresence of water, give rise to a strong decrease in selectivity.

Within the scope of the invention "specific surface area" means the BETsurface area as defined by S. Brunauer et al., in J.A.C.S. 60, 309(1938). Use was made of a nitrogen adsorption at 77K according to theso-called three-point measurement. For the purpose of the calculation,the surface area of a nitrogen molecule was assumed to be 16.2 Å².

The average pore radius is determined starting from a cylindrical poremodel, using the following formula: ##EQU2##

The pore volume used herein is gravimetrically determined byimpregnation with water in vacuo. The specific pore volume can also bedetermined using mercury porosimetry up to a pressure of 2000 bar. Thevalues obtained by the two methods show a good correspondence.

The specific surface area of the catalyst according to the invention cancorrespond with the values according to U.S. Pat. No. 4,818,740 as wellas with the values according to European patent publication 409,353.More particularly, the specific surface area is at least 25 m² /g, sincea good activity can be obtained with such values.

Preferably, the specific surface area of the catalyst will not be largerthan 300 m² /g of catalyst. In general no specific additional advantagescan be gained with higher values.

The requirement as regards the average pore radius is relevant inconnection with the nature of the reaction. Too many small pores involvethe danger of continued oxidation of sulfur to SO₂ due to the sulfurremaining in the pores too long, which is undesirable. The average poreradius is generally preferably at least 325 Å, while 2000 Å is an upperlimit. In general, no additional advantage is to be gained above thislimit, while on the other hand problems may arise in the preparation ofthe support. More particularly, an average pore radius not exceeding 500Å is preferred.

The nature and amount of the alkali metal promoter can vary. In respectof its nature it is observed that it can be based on the conventionalalkali metals, more particularly lithium, rubidium, cesium, potassiumand sodium, the last two materials being preferred most.

The amount depends at least partly on the nature and amount ofcatalytically active material. A preference is expressed for amounts ofalkali metal promoter relative to the amount of catalytically activematerial, both calculated as metal, between 1 and 350 atomic percent,more particularly between 5 and 100 atomic percent.

The catalyst according to the invention generally comprises 0.1-50% byweight, calculated on the total mass of the catalyst, of a materialwhich is catalytically active for the selective oxidation of H₂ S toelemental sulfur.

It should be stressed that this relates to the active material which isaccessible to the reaction gases. In fact, by sintering or by adifferent process of preparation, a part of the active material, inparticular metal oxide, can be encapsulated, for instance by sinteringup narrow pores in the support. However, the difference betweenencapsulated metal oxide and metal oxide present on the support caneasily be determined by temperature programmed reduction (TPR). Detailsof this measuring technique are described in N. W. Hurst, S. J. Gentry,A. Jones and B. D. McNicol, Catal. Rev. Sci. Eng. 24 (2), 233-309(1982). The amount of metal oxide present that is accessible to gasescan thus be determined.

As catalytically active material, effectively a metal compound is used,or a mixture of metal compounds, optionally in combination with one ormore compounds of nonmetals.

Preferably, the catalytically active material used is an iron compoundor a compound of iron and chromium is used. Effectively, a molar ratioof Cr:Fe is chosen which is lower than 0.5 and preferably in the rangeof 0.02-0.3.

The catalyst according to the invention may also contain one or moreother promoting materials. Suitable promoting materials according to theinvention are phosphorus compounds. These can be applied to the catalystinter alia by impregnation with a soluble phosphorus compound.

The catalyst consists of a support material to which a catalyticallyactive material has been applied.

Preferably, the active component is present on the support in an amountin the range of 0.1-40% by weight, more preferably 0.1-10% by weight,calculated on the total weight of the catalyst.

Generally, as a support a ceramic material is used which exhibits noClaus activity under reaction conditions or has been deactivated asregards this activity. Suitable materials include aluminum oxide,titanium oxide and silicon oxide.

However, it is also possible to use, as a support, other materials whichexhibit no or minimal Claus activity and are thermostable. Examples arethermostable non-ceramic materials, such as metal gauze structures andsurfaces of (incompletely) sintered materials. Very suitable is ahoneycomb structure having a high thermal conductivity. Suitablematerials for such supports are the various metal alloys which arestable under the reaction conditions. Examples are metals such as Fe, Cror Ni or alloys containing one or more of these metals.

On the surface of these materials, if desired, a layer of oxidicmaterial may be provided as a support for the catalytically activematerial and which incorporates the alkali metal promoter.

In principle, the catalysts according to the invention can be preparedby the known methods for preparing supported catalysts.

In order to bring the catalyst support into a suitable form, it may, ifnecessary, be subjected beforehand to a sintering treatment beforeand/or after application.

A sintering treatment can optionally be carried out with a finishedcatalyst, whereby micropores are sintered up.

The application of the alkali metal promoter can be effected in a mannerwhich is conventional for the application of components on a supportedcatalyst. Preferably, sequential precipitation is used, i.e. firstprecipitating a precursor of the alkali metal promoter, followed by theapplication of a precursor of the catalytically active component. Asalkali metal compound, it is preferred to start from water solublealkali metal salts, which, after application, are converted into thedesired promoter by calcination. Suitable salts are, first of all,nitrates and carbonates, as well as alkali metal salts of organic acids,such as oxalate, citrate, acetate, and formate.

It is also possible to incorporate the alkali metal promoter orprecursor thereof into the oxidic material during the preparation of thesupport material or the shaping thereof. This can be effected bypreparing the support from starting materials already incorporatingprecursor for the alkali metal promoter. It is also possible to addalkali metal compounds during the shaping of the support material.

It is observed that Example 9 of European patent publication 409,353describes a method in which a coimpregnation of a sodium citrate and aniron compound takes place. As can be derived from the Examples includedhereinafter, such coimpregnation does not provide any advantages withregard to activity and selectivity.

In the preparation of supported catalysts, the homogeneous applicationof the promoting alkali metal and of the catalytically active materialto the support material requires particular care, and furthermore itmust be ensured that this homogeneity is maintained during and after thedrying procedure.

To satisfy these requirements, it is quite eligible in the preparationof such catalysts to utilize the "dry" impregnation of the supportmaterial with a solution of a precursor of the active component orcomponents. This method is known as the so-called incipient wetnessmethod. Good results are obtained with a solution of EDTA or citratecomplexes. An amount of a viscosity increasing compound such ashydroxyethyl cellulose may be added to the solution. By impregnating thesupport material with this solution by means of the incipient wetnessmethod, a catalyst is obtained on which the active material is providedwith a high degree of homogeneity.

The invention also relates to a process for the selective oxidation ofsulfur-containing compounds, in particular hydrogen sulfide, toelemental sulfur, using the catalyst according to the invention.

According to this process, hydrogen sulfide is oxidized directly toelemental sulfur by passing a hydrogen sulfide-containing gas togetherwith an oxygen-containing gas over the catalyst at an elevatedtemperature.

It is noted that not only the nature of the catalyst but also theprocess parameters determine whether optimum results are obtained. Ofparticular importance are the selected temperature and the contact timefor the oxidation. The use of the present catalyst, incidentally,permits tolerating an excess of oxygen and/or the presence of water inthe gas to be treated.

The oxidation process is carried out by adding to the hydrogensulfide-containing gas such an amount of oxygen or an oxygen-containinggas, using a known ratio regulator, that the molar ratio of oxygen tohydrogen sulfide is between 0.5 and 5.0, and preferably between 0.5 and1.5.

The process according to the invention can be used for the selectiveoxidation of all gases containing sulfurous compounds, in particularhydrogen sulfide. Examples of processes where the oxidation according tothe invention can be suitably used are the processes described inEuropean patent application 91551, European patent application 78690 andU.S. Pat. No. 4,311,683.

The process according to the invention is eminently suitable foroxidizing gas which does not contain more than 1.5% of H₂ S, becausethen a normal, adiabatically operating reactor can be used.

In the oxidation the inlet temperature of the catalyst bed is chosen tobe above 150° C. and preferably above 170° C. This temperature is partlydictated by the requirement that the temperature of the catalyst bedmust be above the dew point temperature of the sulfur formed.

One of the advantages of using the invention resides in the fact that aheightened activity is obtained while the selectivity is maintained,which leads to a better sulfur yield. Also, the gas temperature may beinitially lower because the catalyst has a lower initiation temperature.Due to the exothermic character of the oxidation reaction and the factthat if the temperature is too high a non-selective thermal oxidation ofthe sulfur compounds may occur, lowering the initiation temperature isof great importance for increasing the sulfur yield.

By measures known per se, the maximum temperature in the catalyst bed isgenerally maintained below 330° C. and preferably below 300° C.

If the H₂ S content is higher than 1.5 % by volume, it may be necessaryto take measures to avoid the temperature in the oxidation reactorbecoming too high due to the reaction heat released. Such measuresinclude, for instance, the use of a cooled reactor, for instance atubular reactor, where the catalyst is in a tube which is surrounded bya coolant. Such a reactor is described in European patent specification91551. A reactor containing a cooling element may also be employed.Further, it is possible to return the treated gas to the reactor inletafter cooling, whereby an additional dilution of the gas to be oxidizedis attained, or to distribute the gas to be oxidized over a plurality ofoxidation reactors while simultaneously distributing the oxidation airover the various reactors.

According to a particular embodiment of the process according to theinvention, the catalyst is employed as a fluid medium in a fluidized bedreactor, shortcircuiting being prevented by the arrangement of one ormore apertured plates. Thus, optimum heat transfer is obtained.

According to another particular embodiment, the catalyst is utilized inthe form of fixed, for instance honeycomb-like, structures of highthermal conductivity, whereby an undesirable increase in the temperatureof the catalyst is avoided in a suitable manner.

The process according to the invention can be utilized with particularadvantage for the selective oxidation of the hydrogen sulfide-containingresidual gases coming from a Claus plant. Apart from the very highselectivity of the catalyst according to the invention, a very importantadditional advantage thereby obtained is that the removal of water priorto the oxidation is no longer required. If the process according to theinvention is used to oxidize the residual gases referred to, these gasescan preferably be passed first through a hydrogenation reactor, inwhich, for instance, a cobalt-molybdenum containing catalyst is presentand in which all sulfurous constituents are hydrogenated to hydrogensulfide.

According to a variant of the process according to the invention, theselective oxidation stage in which the catalyst according to theinvention is used is combined with a subsequent hydrogenation stage,followed by absorption of hydrogen sulfide, all this as described inEuropean patent application 71983. In the process, 98% of the sulfurcompounds present are thus removed in the part preceding hydrogenation,so that the hydrogenation stage and the absorption mass are not undulyloaded. In this way sulfur recovery percentages of up to 100% can beachieved. According to a variant of this process, it is possible afterthe hydrogenation stage to use, instead of the absorption mass, arenewed selective oxidation according to the invention, whereby a totalsulfur recovery percentage between 99.5 and 99.8% is attained.

Further, the process according to the invention is particularly suitablefor desulfurizing, for instance, fuel gases, refinery gases, biogas,coke furnace gas, gaseous effluents of chemical plants such as viscosefactories, or gases which are burnt off at gas and/or oil extractionsites.

If in the process according to the invention the sulfur vapor-containinggas coming from the selective oxidation stage, optionally aftercondensation and separation of the greater part of the sulfur, is passedover a bed where the sulfur is removed by capillary adsorption, thesulfur recovery percentage is increased to virtually 100%.

The invention is illustrated in and by the following examples.

EXAMPLE 1

100 g Silica (Degussa OX50, B.E.T. 44 m² /g) was mixed with 147 ml waterand 1.5 g HEC (Hydroxy Ethyl Cellulose) and extruded. The extrusionswere dried at 100° C. To obtain sufficient mechanical strength, theextrusions were calcined at 700° C. The preformed support thus obtainedhad a BET surface area of 45.5 m² /g, a pore volume of 1.1 cm³ /g and anaverage pore radius of 350 Å.

EXAMPLE 2

5.23 g ammonium iron citrate (16 wt. % Fe) was dissolved indemineralized water and supplemented to 25 ml. An amount of 10 g of theextrusions obtained in Example 1 was impregnated with 11 ml of the abovesolution. The extrusions were first dried at room temperature for 3hours and then at 120° C. for another 3 hours. The dried extrusions werecalcined in a rotary quartz tube under an air flow of 50 ml/min at 500°C. The catalyst obtained in this manner had a BET surface area of 51 m²/g, an iron oxide content of 5 wt. %.

EXAMPLE 3 (COMPARATIVE)

0.28 g trisodium citrate dihydrate and 5.23 g ammonium iron citrate (16wt. % Fe) were dissolved in demineralized water and supplemented to 25ml. An amount of 10 g of the extrusions obtained in Example 1 wasimpregnated with 11 ml of the above solution. The extrusions were firstdried at room temperature for 3 hours and then at 120° C. for another 3hours. The dried extrusions were calcined in a rotary quartz tube underan air flow of 50 ml/min at 500° C. The catalyst obtained in this mannerhad a BET surface area of 50 m² /g, an iron oxide content of 5 wt. %.The molar ratio of sodium to iron is 1:5.

EXAMPLE 4

0.28 g trisodium citrate dihydrate was dissolved in demineralized waterand supplemented to 25 ml. An amount of 10 g of the extrusions obtainedin Example 1 was impregnated with 11 ml of the above solution. Theextrusions were first dried at room temperature for 3 hours and then at120° C. for another 3 hours. The dried extrusions were calcined in arotary quartz tube under an air flow of 50 ml/min at 500° C. The sodiumpromoted silica thus obtained had a BET surface of 45 m² /g, a sodiumcontent of 0.29 wt. %.

EXAMPLE 5

5.23 g ammonium iron citrate (16 wt. % Fe) was dissolved indemineralized water and supplemented to 25 ml. An amount of 10 g of thesodium promoted silica obtained in Example 4 was impregnated with 11 mlof the above solution. Again the extrusions were first dried at roomtemperature for 3 hours and then at 120° C. for another 3 hours,whereafter they were calcined in a rotary quartz tube under an air flowof 50 ml/min at 500° C. The catalyst obtained in this manner had a BETsurface of 52 m² /g, an iron oxide content of 5 wt. %. The molar ratioof sodium to iron was 1:5.

EXAMPLES 6, 7 AND 8

From the catalysts prepared according to Examples 2, 3 and 5, sievefractions were made with a particle size of between 0.4 and 0.6 mm. Aquartz reactor tube having an internal diameter of 8 mm was filled with1 ml of this catalyst. A gas mixture of the following molar compositionwas passed downflow over the catalyst: 5% O₂, 1% H₂ S, 30% H₂ O in He.The space velocity (Nml of gas per ml of catalyst per hour) of the gaswas 12,000 hr⁻¹. The temperature was raised by steps of 10° C. from 200°C. to 300° C. and then lowered again to 200° C. The sulfur vaporgenerated was condensed downstream of the reactor at 130° C. The watervapor was removed with a water permeable membrane. The composition ofthe influent and the effluent gas was determined with a gaschromatograph. The results of the experiments are summarized in Tables1, 2 and 3.

                  TABLE 1                                                         ______________________________________                                        Example   Temperature                                                                             Act.       Sel. Yld                                       ______________________________________                                        2         200        33        96   32                                        only      220        72        96   67                                        iron oxide                                                                              240        99        94   93                                                  250       100        89   89                                                  260       100        82   82                                                  280       100        60   60                                                  300       100        22   22                                        ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                        Example    Temperature                                                                             Act.       Sel. Yld                                      ______________________________________                                        3          200       17         94   16                                       sodium-iron                                                                              220       39         94   37                                       coimpreg-  240       70         94   66                                       nation     250       94         94   88                                                  260       99         93   92                                                  280       100        88   88                                                  300       100        75   75                                       ______________________________________                                    

                  TABLE 3                                                         ______________________________________                                        Example    Temperature                                                                             Act.       Sel. Yld                                      ______________________________________                                        5          200        35        96   34                                       sodium-iron                                                                              220        64        96   61                                       sequential 240        95        96   91                                       impregnation                                                                             250       100        95   95                                                  260       100        93   93                                                  280       100        84   84                                                  300       100        69   69                                       ______________________________________                                         act. = percentage of converted hydrogen sulfide                               sel. = selectivity to elemental sulfur                                        yld = sulfur yield                                                       

EXAMPLE 8

Of the extrusions prepared according to Example 1, a sieve fraction wasmade having a particle size of between 0.4 and 0.6 mm. A quartz reactortube having an internal diameter of 8 mm was filled with 1 ml of thissample. Placed hereon was 1 ml of sieve fraction of the catalystprepared according to Example 2. Under identical conditions to thosedescribed in Examples 5, 6 and 7, measurements were performed on thissample. The results of this experiment are summarized in Table 4.

                  TABLE 4                                                         ______________________________________                                        Example   Temperature                                                                             Act.       Sel. Yld                                       ______________________________________                                        8         200        33        96   32                                        iron oxide                                                                              220        72        96   67                                        catalyst  240        99        94   93                                        with silica                                                                             250       100        89   89                                        under it  260       100        79   79                                                  280       100        44   44                                                  300       100         0    0                                        ______________________________________                                    

EXAMPLE 9

Of the sodium promoted silica prepared according to Example 4, a sievefraction was made having a particle size of between 0.4 and 0.6 mm. Aquartz reactor tube having an internal diameter of 8 mm was filled with1 ml of this sample. Placed hereon was 1 ml of sieve fraction of thecatalyst prepared according to Example 2. Under identical conditions tothose described in Examples 5, 6 and 7, measurements were performed onthis sample. The results of this experiment coincide exactly with thosesummarized in Table 1.

We claim:
 1. A support-based catalyst for the selective oxidation ofsulfur-containing compounds to elemental sulfur, said catalystcomprisinga) a support material, containing at least one alkali metalpromoter, which is thermostable and substantially inert to the Clausreaction; and b) at least one catalytically active material applied tosaid support material containing at least one alkali metal promoter,said catalytically active material selected from the group consisting ofiron compounds; and a mixture of iron compounds and chromiumcompounds,wherein the average pore radius of the catalyst is at least 25Å, and the amount of said alkali metal promoter relative to the amountof said catalytically active material, both calculated as metal, isbetween 1 and 350 atomic percent.
 2. A catalyst according to claim 1,wherein the alkali metal is selected from the group consisting ofsodium, potassium, rubidium, cesium and lithium.
 3. A catalyst accordingto claim 2, wherein the amount of alkali metal promoter relative to theamount of catalytically active material, both calculated as metal, isbetween 5 and 100 atomic percent.
 4. A catalyst according to claim 2,wherein the catalyst has a specific surface area greater than 20 m² /gand an average pore radius of at least 200 Å, while under the reactionconditions the catalyst exhibits substantially no activity for the Clausreaction.
 5. A catalyst according to claim 1, having a specific surfacearea greater than 20 m² /g and an average pore radius of at least 200 Å.6. A catalyst according to claim 1, having a specific surface areagreater than 25 m² /g.
 7. A catalyst according to claim 1, wherein saidaverage pore radius does not exceed 2000 Å.
 8. A catalyst according toclaim 1, wherein said support material is SiO₂.
 9. A catalyst accordingto claim 1, wherein the amount of said catalytically active material onsaid support is 0.1-10% by weight, calculated on the total mass of thecatalyst.
 10. A catalyst according to claim 1, further comprising one ormore phosphorus compounds.
 11. A process for preparing a catalystaccording to claim 1, comprising the steps of a) impregnating athermostable support which is substantially inert to the Claus reactionwith an alkali metal promoter or precursor therefor, and b) applying atleast one catalytically active material selected from the groupconsisting of iron compounds, and iron and chromium compound mixtures,to said support.
 12. A process according to claim 11, wherein saidimpregnation step is accomplished using the incipient wetness methodwith a solution containing alkali metal ions.
 13. The process of claim11, wherein said catalyst has a specific surface area greater than 25 m²/g and an average pore radius from at least 200 Å to 2000 Å; saidsupport material is SiO₂ ; said catalytically active material is presenton the support in an amount of 0.1-10% by weight, calculated on thetotal mass of the catalyst; and said catalyst contains one or morephosphorus compounds.
 14. A process for the selective oxidation ofhydrogen sulfide to elemental sulfur, comprising the steps of passing ahydrogen sulfide-containing Claus tail gas, together with anoxygen-containing gas, over the catalyst according to claim 1 at atemperature of no greater than 330° C.
 15. A process according to claim14, characterized in that the molar ratio of oxygen to hydrogen sulfideis maintained between 0.5 and 1.5.
 16. The process of claim 15, whereinthe amount of hydrogen sulfide in said hydrogen sulfide-containing Claustail gas is less than about 1.5% by volume.
 17. The process of claim 14,wherein the amount of hydrogen sulfide in said hydrogensulfide-containing Claus tail gas is less than about 1.5% by volume. 18.A process for the selective oxidation hydrogen sulfide to elementalsulfur, comprising the steps of passing a hydrogen sulfide-containingClaus tail gas, together with an oxygen-containing gas, over thecatalyst of claim 23 at a temperature of no greater than 330° C.,wherein the the molar ratio of oxygen to hydrogen sulfide is maintainedbetween 0.5 and 1.5.